A great variety of solar collectors are presently known and in use. The most popular construction is the flat plate collector which is normally employed in solar water heating installations. While the flat plate collector is useful for producing a relatively low temperature heat output in the vicinity of 60.degree.-70.degree. C., at higher temperatures it has extremely low efficiency. As a result, the flat plate collector is not suitable for generating steam.
There exist a number of important applications, such as power generation where the provision of high output temperatures is required. Current technology for producing such temperatures in solar collectors calls for a focussing collector in which an image of the sun is focussed on a solar absorber. Due to the movement of the sun during the day, tracking apparatus is generally required to keep the sun's image on the absorber surface.
Tracking apparatus is relatively expensive and cumbersome and requires continuing maintenance to maintain its operational efficiency. For this reason solar collector apparatus employing mechanical tracking has not gained market acceptance. Also, focussing collectors do not employ diffused radiation.
The present invention seeks to overcome disadvantages of the prior art solar collection apparatus and to provide solar collection apparatus which provides a high temperature output at relatively high efficiency and without requiring solar tracking apparatus, and which employs diffused radiation as well.
U.S. Pat. No. 4,162,824 discloses a non-imaging radiant energy collector and concentrator which employs elongate curved mirrors. U.S. Pat. No. 4,141,340 discloses a solar energy collector employing upstanding elongate absorber panels interdigitated with inclined mirrors. This configuration provides practically no concentration and appears to require a tracking device for efficient operation.
U.S. Pat. No. 4,069,812 describes a solar concentrator and energy collection system which operates on refraction and would appear to require the use of a tracking system for efficient operation. It does not absorb diffuse radiation.
None of the above-described references employ total internal reflection for concentrating solar radiation onto an absorber.
U.S. Pat. No. 4,154,219 describes a prismatic solar reflector and method of solar tracking which defines a focussing collector and employs total internal reflection of a prism for focussing light onto an absorber. The prism is characterized in that light enters and leaves through the same surface. The apparatus does not employ diffuse solar radiation. The prisms are spaced from the absorber surface. A similar disclosure is to be found in "Prisms with Total Internal Reflection as Solar Reflectors" by Ari Rabl in Solar Energy Vol. 19, pp. 555-565, Pergamon Press 1977. These disclosures may be readily distinguished from the present invention as described hereinbelow in that they relate to focussing collectors, rather than to non-focussing collectors which are the subject of the present invention.
U.S. Pat. No. 4,056,094 discloses a solar heat collector which employs total internal reflection for reflection but not for concentration.
U.S. Pat. No. 4,045,246 describes a two-stage concentrator and employs a liquid with an index of refraction higher than that of air. This liquid is not used to provide total internal reflection.
German Offenlegungsschrift No. 2827 708 describes a solar energy concentrator and collector employing total internal reflection but wherein the incident surface is not perpendicular to the absorber surface. The configuration shown in this German Offenlegungsschrift is designed to provide the highest concentration ratio of the area of the incident surface to the area of the absorber surface. It does not, however, enable a back-to-back construction utilizing both surfaces of an absorber plate and thus requires, for a given area, twice as many absorber plates as are required in the present invention. Additionally, significant heat losses are encountered due to disclosed construction wherein the back surface of the absorber plate is open to the atmosphere.
U.S. Pat. No. 4,024,852 shows a solar energy reflector collector including an absorber plate whose two surfaces define solar energy absorbing surfaces. The structure employed is that of an elongate collector of parabolic cross section.
U.S. Pat. No. 4,003,364 discloses a solar heating system having a complex cross section including fins having a plurality of surfaces and defining a fluid path therethrough.